Gases such as nitrogen and oxygen are the most widely and heavily used gases in industry. These gases are typically produced using a cryogenic air separation method in which air is cooled, liquefied, and then separated by distillation.
However, if the air contains components that solidify at the temperatures used during liquefaction, then the fluid passages can become plugged, making operation of the unit impossible. In order to avoid such problems, the air is generally treated by a pre-purification unit to remove these plugging components prior to cooling of the air. The main plugging components removed by this type of pre-purification unit are water and carbon dioxide.
Thermal Swing Adsorption (TSA) is the most widely used method of pre-purification, and has been reported in numerous documents and patents. In a typical TSA pre-purification unit, the column is packed with layers of two different adsorbents, with activated alumina used for removing water in the upstream portion of the column, and a synthetic zeolite used for removing carbon dioxide in the downstream portion of the column. Considering factors such as the quantity of carbon dioxide adsorbed at low partial pressures and the cost of the adsorbent, NaX zeolites are the most widely used synthetic zeolites.
However, in recent years it has become clear that in order to ensure the safety of cryogenic air separation units, components such as dinitrogen monoxide and hydrocarbons must be removed in addition to the components described above.
Air contains approximately 0.3 ppm of dinitrogen monoxide, and this dinitrogen monoxide acts as a plugging material in a similar manner to water and carbon dioxide. Conventionally, because of its low concentration within feed air, dinitrogen monoxide has not been considered as a component that requires removal, but as a result of increases in the dinitrogen monoxide concentration in the atmosphere, as well as changes in gas behavior within cryogenic air separation units as a result of improvements, modifications and performance gains within these units, dinitrogen monoxide is now considered a component that should be targeted for removal.
The hydrocarbons which are contained in air are mainly light hydrocarbons of 1 to 3 carbon atoms, and specific examples include methane, acetylene, ethylene, ethane, propylene, and propane. The concentration of methane in air is comparatively high (approximately 1.6 ppm). The other hydrocarbon components exist only in trace quantities in the order of ppb. These hydrocarbons dissolve in, and become concentrated within liquid oxygen, and because they can cause combustion or explosions within the unit, the hydrocarbon concentration within the liquid oxygen needs to be controlled using the solubility and the explosive range as indicators. Specifically, liquid oxygen in which the hydrocarbons have been concentrated is discharged from the unit in a liquid oxygen purge in order to ensure that the hydrocarbon concentration within the liquid oxygen does not exceed a certain level. The hydrocarbon concentration within this purged liquid oxygen is prescribed by law.
However, as a result of the aforementioned types of changes in gas behavior within cryogenic air separation units as a result of improvements, modifications and performance gains within these units, the possibility of localized concentrating of hydrocarbons in locations outside of the liquid oxygen sump into which the purged liquid oxygen is extracted cannot be ruled out. From the outset, the introduction of components that are likely to jeopardize the safety of the unit is not at all desirable, and hydrocarbons should preferably also be removed at the pre-purification stage.
Reyhing et al. showed that with conventional pre-purification units, although propylene and acetylene could be removed, dinitrogen monoxide and other hydrocarbons could not be completely removed. The results of recent investigations conducted by cryogenic air separation unit makers into adsorbents capable of removing these other components are disclosed in the document: (Linde Reports on Science and Technology, 36/1983, Dr J. Reyhing).
Japanese Unexamined Patent Application, First Publication No. Hei 11-253736 discloses that ethylene can be removed from air using an X or LSX zeolite that has undergone ion exchange with calcium.
Japanese Unexamined Patent Application, First Publication No. 2000-140550 discloses that dinitrogen monoxide and ethylene can be removed from air using an X or LSX zeolite that has undergone ion exchange with calcium.
Japanese Unexamined Patent Application, First Publication No. 2000-107546 discloses that dinitrogen monoxide and ethylene can be removed from air using a binderless X zeolite that has undergone ion exchange with calcium.
Japanese Unexamined Patent Application, First Publication No. 2001-62238 discloses that dinitrogen monoxide, ethylene, and propane can be removed from air using an A or X zeolite that has undergone ion exchange with calcium.
Japanese Unexamined Patent Application, First Publication No. 2002-126436, Japanese Unexamined Patent Application, First Publication No. 2002-143628, and Japanese Unexamined Patent Application, First Publication No. 2002-154821 disclose that dinitrogen monoxide and hydrocarbons can be removed using a Ca-LSX, Ca-A composite adsorbent.
Japanese Unexamined Patent Application, First Publication No. 2002-143677 discloses that dinitrogen monoxide and hydrocarbons can be removed from air using a binderless LSX zeolite that has undergone ion exchange with calcium.
Japanese Unexamined Patent Application, First Publication No. 2001-129342 discloses that dinitrogen monoxide and ethylene can be removed from air using an LSX zeolite that has undergone ion exchange with calcium.
The adsorbents of these patent documents are X (LSX) type zeolite or A type zeolite which are ion exchanged with calcium.
Calcium ion exchange is effective for adsorbing components that mainly a specific mutual interaction, including dinitrogen monoxide and particular hydrocarbons such as ethylene. Said zeolites, however, are thought to make no significant contribution to the adsorption of components such as propane that exhibit no such specific interaction. Indeed, in most of the patent documents described above, the only components for which significant effects were observed in the examples were dinitrogen monoxide and ethylene.
Amongst the various hydrocarbons, current technology allows ethylene, acetylene, and propylene to be removed comparatively easily, but the remaining hydrocarbons such as methane, ethane, and propane (all of which are saturated hydrocarbons) cannot be efficiently adsorbed and removed.
Testing involving the removal of hydrocarbons by adsorption is also being conducted in the field of vehicle exhaust gas treatment. The exhaust gas from vehicles is generally treated using a catalyst.
Usually, the temperature of the catalyst immediately following engine startup is low, meaning the catalytic activity is also low, and consequently the exhaust gas is discharged into the atmosphere without treatment. Accordingly, for the period until the temperature of the catalyst rises sufficiently to increase the catalytic activity, these hydrocarbon emissions are prevented from being discharged by temporarily adsorbing the hydrocarbons within the exhaust gas in a zeolite trap that is provided in a separate preliminary stage. Subsequently, when the temperature of the exhaust gas increases, the hydrocarbons adsorbed to the zeolite desorb, and are treated by the latter stage catalyst. Alternatively, in those cases where the zeolite trap itself exhibits catalytic action, treatment may also occur within the trap.
Existing technology within this field includes, for example, that described in Japanese Unexamined Patent Application, First Publication No. 2001-293368, which discloses that a zeolite containing an alkali metal such as Cs and with a SiO2/Al2O3 ratio of 10 or greater is effective in the treatment of exhaust gas from an internal combustion engine. This document discloses that toluene can be adsorbed using a Cs-ZSM5 or K-ZSM5 adsorbent. Furthermore, because the treatment target is the exhaust gas from an internal combustion engine, a larger SiO2/Al2O3 ratio is reported as being more favorable in terms of preventing desorption at low temperatures and ensuring favorable heat resistance.
Japanese Unexamined Patent Application, First Publication No. 2003-126689 discloses that a zeolite in which the SiO2/Al2O3 ratio is 30 or greater and the absolute value of the oxygen charge is 0.210 or greater is effective in the treatment of exhaust gas from an internal combustion engine.
Japanese Unexamined Patent Application, First Publication No. 2001-293368 and Japanese Unexamined Patent Application, First Publication No. 2003-126689 list the following conditions as the main usage conditions under which a hydrocarbon-adsorbing zeolite is used within the field of vehicle exhaust gas treatment.
(1) The exhaust gas contains a comparatively large quantity of water.
(2) The temperature of the exhaust gas is 600° C. or higher (and during high speed operation may be 1,000° C. or higher).
(3) Hydrocarbons will not desorb until a suitably high temperature is reached.
(4) Even at low estimates, hydrocarbon concentration is several dozen ppm, and in practical examples is approximately several thousand ppm.
Zeolite adsorbs water molecule with high polarity preferentially, so the hydrocarbon adsorption performance tends to deteriorate. Accordingly, an adsorbent that exhibits excellent hydrocarbon adsorption performance even in the presence of water is keenly sought. Generally, zeolites in which the Si/Al ratio is low are more strongly affected by the presence of water, and consequently in the field of vehicle exhaust gas treatment, zeolites with high Si/Al ratios tend to be used.
Because they are used at high temperatures, the zeolites must exhibit high levels hydrothermal resistance. Although dependent on the variety of zeolite used, zeolites are generally said to be prone to structural breakdown under conditions of high temperature and water. Generally, zeolites with higher Si/Al ratios tend to exhibit higher levels hydrothermal resistance.
If the activity of the catalyst at a subsequent stage to the adsorbent is not raised by increasing the temperature, then the gas containing the hydrocarbons cannot be treated, and consequently a zeolite that is able to retain (adsorb) the hydrocarbons until a high temperature is reached is required.
As described above, unlike the conditions required for zeolites used in the treatment of vehicle exhaust gases, the usage conditions for zeolites used in TSA units that function as the pre-purification units for cryogenic air separation units are as follows.
(1) Air with a lower water content than an exhaust gas is purified.
(2) Adsorption occurs at normal temperatures of 5 to 40° C., and regeneration occurs at a comparatively low temperature of 100 to 300° C.
(3) The zeolite does not require high levels hydrothermal resistance.
(4) The hydrocarbon concentration within the air (with the exception of methane) is in the order of several dozen ppb.
In a TSA unit, air with a comparatively low water content is treated. Furthermore, if an adsorbent exhibits particular weakness in the presence of water, then by using the adsorbent downstream from another adsorbent that is used for removing water, hydrocarbon adsorption can be conducted in the absence of water. Moreover, in terms of factors such as running costs, the regeneration temperature is preferably kept as low as possible. In other words, in complete contrast to the adsorbents used in the treatment of vehicle exhaust gases, which must adsorb and retain the hydrocarbons right up to high temperatures, the hydrocarbon adsorbent in a TSA unit should desorb readily at low temperatures. Furthermore, because hydrothermal resistance is not required, there is no need to increase the Si/Al ratio.